Considerable efforts have been made in recent years to develop new devices utilizing photonic technology, since photonics offers features that cannot be duplicated by conventional electronics. For example, photonics is well established in long distance telecommunications as exemplified by the extensive use of fiber-optic cable. More recently, efforts have been made to derive the advantages of photonics in the area of information processing.
Presently, the field of information processing is dominated by electronics. While electronics combines speed, control, and precision with low cost, it has shortcomings in the bandwidth (amount of information) that can be carried on an electronic channel, and it suffers from susceptibility to electromagnetic interference. Although bandwidth can be effectively increased by implementing many side-by-side electronic channels, this approach requires that special attention be paid to isolating each channel from the others, resulting in increased size and higher cost. The result is that electronics suffers from a mismatch between the speed of handling information within a processor system and the rate of sending information between processors or from a processor to an outside user. Thus, there is a communications bottleneck inherent in electronics. Photonics offers the possibility of overcoming this bottleneck and greatly expanding the performance capability of information-processing systems.
In attempts to overcome the inherent bottleneck of electronic systems, a number of technologies have been demonstrated for modulating an optical signal using electronic signals. These include mechanical devices that physically move fibers or that physically move lenses or mirrors directing an optical beam. These devices are difficult to fabricate and have been essentially limited to special purpose applications where the high costs of such devices can be justified. An alternative approach is to form waveguides from materials whose optical properties can be controlled in order to modulate light propagating through the material. In many materials, their optical properties (specifically the refractive index) can be altered under the application of an electric field using an effect that is commonly referred to as the "linear" electrooptic effect. By identifying the electrooptic effect in certain materials as "linear", it is distinguished from other electrooptic effects in other materials which stem from different underlying physical phenomenon, as discussed hereinafter. Ferroelectrics are an example of a type of material having a strong linear electrooptic effect and good transparency in both the visible and near infrared spectral regions. A large dielectric polarizability characterizes ferroelectrics and leads to their large linear electrooptic effect. Described mathematically, the predominant term of the electrooptic effect in materials such as ferroelectrics is linear with respect to the strength of an applied electric field--hence the name "linear" electrooptic effect. This large linear electrooptic effect in ferroelectrics has been used in various ways to construct waveguide devices which are capable of modulating an electrical signal onto light. Some examples include phase modulators, directional-coupler switches and Mach-Zehnder interferometers.
A different and unusual use of this linear electrooptic effect is in a voltage-induced optical waveguide modulator. Such a device was first proposed and demonstrated by D.J. Channin ("Voltage-Induced Optical Waveguide," Applied Physics Letters, Vol. 19, No. 5, pp. 128-130, 1971), but because of fabrication difficulties, no practical devices have been constructed using this technique. In this device, voltage is applied to a pair of coplanar electrodes separated by a small gap on an electrooptic substrate made from a ferroelectric such as lithium niobate (LiNbO.sub.3). The resulting electric field induces a change in the refractive index of the substrate that forms an optical waveguide which allows modes of propagation in the inter-electrode gap region.
Semiconductor compounds formed from combinations of the Group III and V elements and from combinations of the Group II and VI elements also exhibit a change in their optical properties in response to the presence of an electric field. For example, gallium arsenide (GaAs) and aluminum gallium arsenide (Al.sub.x Ga.sub.1-x As) and the quatinary compound aluminum gallium indium arsenide ((Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y As) exhibit the linear electrooptic effect, although it is substantially weaker than in some ferroelectrics (e.g., lithium niobate). Unlike the ferroelectrics, however, an electric field applied to bulk GaAs and Al.sub.x Ga.sub.1-x As can produce changes in the refractive index from two additional mechanisms. One mechanism results from free carrier absorption in semiconductor material. An increase in free carriers in the material results in a decrease in the refractive index. The application of an electric field can result in a change in the free carrier density, thereby altering the refractive index. The second mechanism, known as electrorefraction, occurs for light with photon energies just below the fundamental energy band gap of the material. The application of an electric field alters the characteristics of the optical absorption of the energy band gap, thereby altering the refractive index.
Devices using the linear electrooptic effect have had very limited commercial success. One reason for this lack of success is the relatively large electric field and corresponding large voltage required to induce the necessary change in the refractive index. Some electrooptic materials such as LiNbO.sub.3, however, have strong linear electrooptic effects, but they are typically passive materials, meaning that they cannot be fabricated into light emitting and/or detecting devices, thereby limiting their potential for providing monolithically integrated optical circuits. Moreover, the propagation and polarization of light through these electrooptic materials is dependent upon the direction of propagation with respect to the crystal lattice of the material, thereby adding severe design constraints to any device fabrication process. Specifically, the linear electrooptic effect is non-isotropic, and only particular relative orientations of the crystal, the applied electric field and the polarization/propagation direction of the light will necessarily exhibit the needed change in index in order to be used in a waveguide. Thus, both the orientations of the crystal substrate and the polarization of the waves to be guided must be carefully considered in the design of an electrooptic waveguide. Such design constraints have presented major obstacles to achieving commercially practical fabrication techniques for fully integrated electrooptical circuits.